1. Field of the Invention
The present invention relates to a method of detecting the inclination angle of the image frame of the image read by a microfilm reader.
2. Prior Art Statement
The document images photographed on a microfilm include some images which have been photographed in inclined postures. When one of such images photographed in inclined postures is projected on a screen, the projected image is also inclined. In reading such an image, the inclined projected image must be rotated by an image rotating prism which is assembled in the optical system of the microfilm reader.
Accordingly, the known microfilm reader has means for automatically detecting the inclination angle of the projected image and rotating the projected image to correct the posture of the projected image. The inclination angle of the projected image is determined by determining the inclination angle of the rectangular image frame, i.e. the contour of the coverage of the document image. For example, when the image photographed on the microfilm is a negative image, the background portion in each image frame is blackened and a white frame region constituted by white picture elements surrounds each image frame. Thus, the inclination angle of each image frame can be determined by detecting the inclination angle of one of the four sides of rectangular image frame, which are boundaries between the image region and the white frame region.
In the conventional system, the negative image projected on the screen is scanned along the main scanning line to read the picture elements continuously. In detail, when black picture elements are sensed continuously it is discriminated that the scanned range is within the image region. On the contrary, when white or bright picture elements are sensed continuously, it is discriminated that the scanned range is out of the image region, i.e. within the white frame range. The coordinates of the boundary separating the continuous white picture element range from the continuous black picture element range are determined to discriminate the image frame. The inclination angle of each image frame is detected by measuring the inclination angle of one of four straight image framing lines.
In the conventional system, the inclination angle of each image frame is determined by detecting the inclination angle of the top side line of the image frame. For instance, the coordinates of both end points of the top side line are determined to set up the equation showing the straight line connecting the end or corner points, or the top side line is determined by the least squares method using the coordinates of the points located on the top side edge or line of the image frame.
However, original documents are usually stacked and stapled by a stapler, so that the upper left or upper right corner of each original document is often damaged. It is a common practice that the original documents are stacked while the bottom side edges of the documents are aligned, for example, on the surface of a desk. Accordingly, the original document stack is stored while the bottom side edges of the thus stacked original documents are aligned. As a result, the bottom side edges of the original documents are protected from being injured, while the top side edges of the original documents tend to be injured. When the inclination angle of the photographed image is determined by detecting the inclination angle of the top side line or edge of the image frame, the accuracy in detecting the inclination angle becomes poor.